The present invention relates broadly to an electromagnetic waveguide network, and in particular to an equalizer cavity apparatus with independent amplitude control.
Distortion, such as phase or delay, deteriorates the performance of communications systems. A common method of improving the performance of a communication system is to compensate for the delay over the narrow band of the system i-f chain with an equalizer unit. For example, if the system must operate anywhere within a broad shf band, such as the military satellite band, then the equalizer unit must be readjusted in the i-f channel every time the carrier frequency is moved appreciably. The group delay varies over the shf band due to the predictable behavior of the various band pass and band reject filters in the transmit or receive chains of the system. A fixed equalizer unit can be designed once and for all to compensate these delay variations over the whole shf band and then the i-f equalizer need not be readjusted each time the carrier frequency is changed. The system now has frequency agility.
A dual mode resonator coupled to a waveguide and connected to a microwave network can equalize typical distortion in the shape of the time delay vs. frequency characteristic of electromagnetic wave energy traveling through the network. However, it is necessary for the proper response of the dual mode resonator that the two modes of the cavity are at the same resonant frequency. It is also necessary that the two modes of the cavity are of equal amplitude and in phase quadrature with each other in order to introduce a transmission coefficient that complements the time delay vs. frequency characteristic of the network, and thereby does not introduce any mismatch.
As simple and as useful as such a component appears to be, it has been found in practice that it is impossible to precisely estimate in advance the time delay vs. frequency characteristic to be equalized and then to specifically design a cavity and its coupling mechanism to provide this characteristic. In other words some adjustment of each equalizier for each application is essential. This requires a continuous adjustability of the loaded Q of the cavity without introducing losses due to reflections at the aperture. No method or apparatus suitable for this purpose is presently known in the art.
There are system components which produce group delays that are not predictable. These system components include waveguide transmission lines whose exact lengths may be unknown until installation; also high powered amplifier tubes, parametric or tunnel diode amplifiers, and similar components may be included in the unknown, unpredictable category. If the system is to change carrier frequency without the need for re-equalization, then the sum of the delays contributed by these components must be carefully measured in the system installation. A variable equalizer would be required as part of such a system. The equalizer unit would be required to provide a wide range of delay shapes, with adjustability to flatten the delay over the entire band.